The present invention relates to an arrangement for the mass-spectrometric detection of ions in the presence of a disturbing background.
In ion detection arrangements, disturbances are caused by foreign particles, intruding into the entrance area of a mass spectrometer, from radiation sources such as ionization gauges, electron-beam heating devices, hot cathodes, or the like. For example, in secondary-ion mass spectrometry, a target is bombarded with primary ions and the secondary ions knocked off by the bombardment are mass-spectrometically analyzed. At the same time, in addition to the secondary ions to be detected, reflected primary ions, high-energy secondary electrons and ions, photons and neutral particles appear as disturbing particles. The neutral particles are disturbing because they may produce again, in secondary processes, charged particles to which the detector is responsive.
To eliminate the disturbing background, various arrangements have already been designed comprising a mass-spectrometric separating system and a detector for determining the presence of ions and in which, along the path of the ions from the source of the detector, means are provided for producing electric or magnetic fields serving to sort out the undesirable particles (RSI, Vol. 44, No. 4, April 1973, pp. 487 to 491).
For example, in a known device (German Offenlegunsschrift No. 2,255,302), a mask interrupting the sight path is mounted between the sample delivering ions and the entrance plane of the mass analyzer, preventing the disturbing particles from passing directly into and through the mass analyzer. In this case, the electrostatic lens must be designed so that a part of the ions to be detected flows past the mask. However, another part of the ions cannot be prevented from being intercepted by the mask and thus is lost for the analysis. Further, the useful ions leave the lens arrangement under a too large inclination relative to the axis of the lens, which is unfavorable for the mass-spectrometric analysis.
In another known arrangement (International Journal of Mass Spectrometry and Ion Physics, 11, (1973) 23-35), a plate capacitor is used for producing an electric deflection field deflecting the useful ions into the mass spectrometer and preventing the disturbing particles from passing thereinto. This arrangement, however, has no direction focusing effect (collecting effect) on the useful ions, so that the desired signal is needlessly weakened.
In a further known arrangement, electrons which are particularly disturbing in the detection of negative ions are separated by means of a magnetic deflection field, whereby a signal-to-background ratio is obtained which is almost equally satisfactory to that obtained hitherto only in the detection of positive ions (German Utility Model No. 73 37645).